Circuit Operation

The proximity switch can work for a wide range of
power, from 8v to 18v DC, D3 protects reverse power
supply connections, and U1 regulates the supply to
+5v , -5v is derived from U2 555 oscillator which
serves dual purpose.

IC 555 is used as an astable oscillator and it
flashes the Infra red LED D1 at a high speed, The
object close to this LED reflects the light along
with the ambient light which may also be sunlight.
The infra red diode D2 detector gets the reflected
light from LED and some ambient light, The forward
voltage drop of D2 will vary with the amount of
light falling on it. Ambient light causes a DC
component and the pulsing light from D1 causes an AC
component.Optical Transmitter IR

The capacitor C6 blocks DC and only transfers AC
pulses if any to opamp amplifier U3A whose gain is
set by R18, D9 rectifies the pulses to DC and this
DC voltage is used by opamp comparator U3B which
drives Q1 through Q2 for an open collector output
for relays. LED D7 turns on when relay Output is
high.

R14 and R13 can be replaced with potentiometer for
threshold adjustment if required.

Testing

Connect 12v DC supply to +V and GND Ports, Connect a
relay coil Between OUT and GND Ports, you can use
the relay contacts as you require to turn on a lamp,
heater, fan or motor.

If all connections are ok and ICs are working you
should see a +5V at U3 pin8 VCC and around -4 to -5
at U3 pin4 VDD

Optical Receiver IR

Construction

The Optic switch can be used for both reflecting
detection (retro reflective) or obstacle
detection. The mechanical construction will decide
this, for obstacle detection the diodes D1 and D2
could be put in two different tubes and can be
kept far apart 2mts+ and both should be exactly
opposite each other, any obstacle like a passing
person will be detected. To make a retro
reflective proximity switch this circuit is ideal,
it can be housed in a cylindrical 30mm by 70mm
metal unit with m30 threads and nuts for mounting,
both D1 and D2 have to be fitted in the front of
this tube on a plastic plug optically insulated
from each other yet beside each other.

IR Led's and Diodes

The types available are various and polarity hard
to detect even photo IR transistors can be used.
The IR Led can be tested in diode mode of a DMM
(battery should be in good condition) it should
give around 1.1V drop in proper polarity. An IR
detector diode or photo diode can be tested in the
same way the drop will be 0.5V at 1 feet from a
60W lamp (no sunlight), closing the IR photo diode
with your hand will be an over range on DMM this
will happen on proper polarity. the photo diode
shows around 10k ohm resistance in daylight and in
Mega ohms when covered also the photo diode
detects light on reverse bias and used like that.

This is a Hand Crafted PCB Artwork done by a PCB
Vendor years back, this method may be used
even today.

This Layout may have many jumpers and may need to be
cleared of Hairline shorts which has happened after
i scanned it, cleaned it and enhanced it using image
manipulation software.

This PCB would go into a 30mm Nickel Plated Brass
threaded tube, with Epoxy or Teflon ends. These were
turned components as quantities made were small. An
Optical Proximity Sensor that would fit the same
place as the eddy current or Inductive sensors, in
existing machines. It could work a longer distance
and could detect Non-Metals and even translucent
fluids.

You can design your own PCB with any PCB Layout
Editor - EDA CAD
Circuit Design Software Tools. Laser prints of
output works well, but even Ink Jet printouts will
work fine for small PCB's like this. Take 3x or 4x
prints.